The invention relates to assay elements suitable for use in automated analytical test systems and, more particularly, to an assay element which includes a thin porous member.
Various types of chemical tests can be performed by automated test equipment, an example of testing of considerable interest being the assay of biological substances for human health care. Automated test equipment allows large numbers of test samples to be processed rapidly. Such equipment is employed in health care institutions including hospitals and laboratories. Biological fluids, such as whole blood, plasma or serum, are tested to find evidence of disease, to monitor therapeutic drug levels, etc.
In the automated test equipment, a sample of the test fluid is typically provided in a sample cup and all of the process steps including pipetting of the sample onto an assay test element, incubation and readout of the signal obtained are carried out automatically. The test instrument typically includes a series of work stations each of which performs one of the steps in the test procedure. The assay element may be transported from one work station to the next by various means such as a carousel to enable the test steps to be accomplished sequentially.
One type of assay element which can be used in an automated analytical is that which includes a thin porous member such as a fibrous mesh pad which is utilized as the site where the reactions(s) and/or interaction(s) occur. Such assay elements may also include reservoirs for storing reagents, mixing fluids, diluting samples, etc. The assay elements also include an opening to permit administration of a predetermined amount of a sample fluid, and if necessary any other required reagent(s), to the porous member such as by a pipette. The assay element may also include a window to allow a signal obtained as a result of the process steps, typically a fluorescent or a colorimetric change in the reagents present in the porous member, to be read such as by means of a spectrophotometer or a fluorometer included in the instrument.
The reagent reservoir(s) in such assay elements may be sealed with a frangible or puncturable thin foil layer so as to provide for a self contained element in which the requisite test reagents are carried by the element itself. The automated analytical test instrument includes the necessary pipette(s) and apparatus for positioning and operating the pipette(s) so as to transfer the sample fluid and other reagents to a mixing reservoir and/or to the porous member so as to permit the reaction(s) and/or interaction(s) upon which the assay is based to take place there. Temperature control means within the analytical test instrument allows for the incubation of the test sample at the temperature required for the assay procedure.
Of particular interest is an assay element which has a thin porous member supported within a guide defined by surfaces contiguous the porous member, the porous member extending between two chambers defined by a housing. The first chamber serves as a dispenser for fluid which is to be applied to the porous member and the second chamber serves as a collector of fluid displaced from the porous member and the guide which supports it. The dispensing chamber can be utilized to apply any fluid required in the assay to the porous member including sample fluids, reagent solutions, wash fluids, etc. During a test step wherein a fluid is applied to the porous member such as where the member is washed to remove excess unused reagent(s), the dispenser is filled with the wash fluid which then travels, via a port in the bottom of the dispenser, along the porous member thereby forcing any unused reagent(s) together with the fluid previously present from the porous member and the support guide into the collecting chamber. The porous member is on the order of about one-half a millimeter in thickness and the top and bottom surfaces of the guide which holds it in place are spaced apart about the same, or a very slightly larger, distance. These dimensions of the thin porous member and its degree of capillarity are such as to induce fluid deposited thereon to be transported throughout the member by capillary action.
A problem arises in the fluid application step(s) due to the capillary action in the transport of the fluid. It has been found that some of the fluid, upon exiting the port at the bottom of the dispenser, can propagate along an interface between a surface of the porous member and a wall of the guide contiguous that surface. This propagation appears to be due to capillary action and surface tension of some of the fluids.
This problem can be particularly acute in the case of a wash fluid which is applied to the porous member to remove excess unused reagents therefrom. Because some of the wash fluid passes along the surface of the porous member and is not available to remove excess unused reagents located within it, an inaccurate test result may be obtained.
Accordingly it is an object of this invention to provide an assay element which includes a porous member and which does not suffer from the aforementioned problem.